This invention relates to a drilling method and apparatus, and in particular to a method and apparatus for drilling through a stratum containing gas hydrates.
The hydrates of hydrocarbon gases have long presented serious problems in the drilling of oil or gas wells. Recently, naturally occurring gas hydrates have been found in polar drilling and in wells drilled under very deep water in temperate or equatorial regions. When such naturally occurring hydrates are encountered and drilling proceeds deeper, the hydrate bearing section may be exposed to increased temperature which causes melting and consequently gas is introduced to the mud column normally used to clean the hole of cuttings and for lubricating the drill bit. The presence of gas in the mud column reduces hydrostatic pressure at the hydrate bearing interval which causes additional melting of hydrates in the vicinity of the hole. The reduction in hydrostatic pressure of the mud column tends to create a condition in which any free gas exposed to the hole will enter the hole. When such a condition occurs, the usual procedure is to take the routine steps for preventing a blowout, i.e., the blowout preventer is closed, gas is circulated out of the system, and the mud density is increased to a level intended to inhibit the flow of additional gas into the hole.
The varying of drilling mud density according to the quantity of gas encountered during drilling is discussed fully, for example, in U.S. Pat. No. 3,498,393, which issued to A. G. West et al on Mar. 3, 1970. The West et al solution to the problem of increased gas production is to continuously monitor the flow rate of gas separated from the drilling fluid (mud), and to adjust the density of the drilling fluid accordingly. When gas pressure increases, the density of the mud is increased and vice versa. U.S. Pat. No. 3,701,388, which issued to J. N. Warren on Oct. 31, 1972, proposes essentially the same solution to the problem of increased gas in the drilling mud. Warren also teaches the use of a temperature detecting method to determine the nature of the formation being drilled.
The present inventor has effected a detailed study of drilling in hydrate-containing strata. An examination of the hydrate pressure/temperature relationship reveals that increased mud density can provide temporary relief from the melting of hydrates, but high pressures are rapidly generated by relatively small increases in temperature. Higher mud density is not a viable method of permanently stopping hydrate melting. Another solution to the gas inflow problem is the cooling of the mud to the lowest possible temperature while increasing mud density. The success of the procedure has not been remarkable. Problems continue to plague the driller, often creating a requirement for multiple casing strings and thus increased expense.
The object of the present invention is to at least partially solve the problem of dealing with naturally occurring hydrates by providing a simple drilling method which facilitates the drilling through a hydrate-containing stratum.